Figure 10 shows the condition of an off channel with V+

and V- present. As with Figures 8 and 9, either an N-

channel or a P-channel device will be off for any input

voltage from -40V to +40V. The leakage current with

negative overvoltages will immediately drop to a few

nanoamps at +25°C. For positive overvoltages, that

fault current will initially be 10µA or 20µA, decaying

over a few seconds to the nanoamp level. The time

constant of this decay is caused by the discharge of

stored charge from internal nodes and does not com-

promise the fault-protection scheme.

Figure 11 shows the condition of the on channel with

V+ and V- present. With input voltages less than ±10V,

all three FETs are on and the input signal appears at

the output. If the input voltage exceeds V+ minus the

will turn off. For voltages more negative than V- minus

3V, the multiplexer’s output swing is limited to about -12V

to +13.5V with ±15V supplies.

Switching Characteristics

and Charge Injection

Table 1 shows typical charge injection levels versus

power-supply voltages and analog input voltage. The

charge injection that occurs during switching creates a

voltage transient whose magnitude is inversely propor-

tional to the capacitance on the multiplexer output.

Fault-Protected Analog Multiplexers

_______________________________________________________________________________________

9

G

D

Q1

S

-25V

N-CHANNEL MOSFET

IS TURNED ON

-25V

OVERVOLTAGE

P-CHANNEL

MOSFET IS OFF

G

D

Q2

S

G

D

Q3

S

Figure 8. -25V Overvoltage with Multiplexer Power Off

+15V

-15V

-15V

Q1

N-CHANNEL MOSFET

IS TURNED ON

-25V

OVERVOLTAGE

P-CHANNEL

MOSFET IS OFF

N-CHANNEL

MOSFET IS OFF

+15V FROM

DRIVERS

-15V FROM

DRIVERS

+25V FORCED

ON COMMON

OUTPUT LINE BY

EXTERNAL CIRCUITRY

Q2

Q3

Figure 10. -25V Overvoltage on an Off Channel with

Multiplexer Power Supply On

G

D

Q1

S

N-CHANNEL MOSFET

IS TURNED OFF

+25V

OVERVOLTAGE

G

D

Q2

S

G

D

Q3

S

Figure 9. +25V Overvoltage with Multiplexer Power Off

+15V

-15V

-15V

Q1

N-CHANNEL MOSFET

IS TURNED OFF

+25V

OVERVOLTAGE

13.5V

N-CHANNEL

MOSFET IS ON

-15V FROM

DRIVERS

+15V FROM

DRIVERS

13.5V

OUTPUT

Q2

Q3

Figure 11. +25V Overvoltage Input to the On Channel

Table 1. MAX354 Charge Injection

analog input level is applied to channel 1; channels 2–8 inputs

a 2kHz rate between 0V and 3V. +100pC of charge creates a

+100mV step when injected into a 1000pF load capacitance.

Supply Voltage

Analog Input Level

Injected Charge

±5V

+2V

0V

-2V

52pC

35pC

16pC

±10V

+5V

0V

-5V

105pC

65pC

25pC

±15V

+10V

0V

-10V

180pC

80pC

15pC